FIG. 9 is a fragmentary sectional view of a conventional semiconductor device. An assembly process of a conventional semiconductor device will be described. An insulated circuit board includes a back copper foil 3, a ceramic 4, and circuit patterns 5 and 6. The back copper foil 3 and a copper base 1 are bonded together by a solder 2 and the circuit pattern 5 and a semiconductor chip 8 are bonded together by a solder 7. The semiconductor chip 8 is a switching element such as an IGBT or a FWD (Free Wheeling Diode).
Usually these solder bonding steps are performed by a single heating process. After that, an emitter electrode (not illustrated) formed over the semiconductor chip 8 is connected to the circuit pattern 6 with a bonding wire 9 (aluminum wire, for example) by ultrasonic vibration.
A terminal case 10 in which external terminals 11 are insert-molded and the copper base 1 are then heat-bonded together with a silicone type adhesive (not illustrated). After that, the circuit pattern 5 and an external terminal 11 are bonded together at welding portions 12 by spot laser welding and the circuit pattern 6 and an external terminal 11 are bonded together at welding portions 12 by spot laser welding. In this laser welding, the upper side of each external terminal 11 is irradiated with laser light. The surface of the semiconductor chip 8 is then covered by injecting resin 17. The conventional semiconductor device is fabricated in this way.
Furthermore, a semiconductor element mounting board in which a board over which a wiring pattern for mounting a semiconductor element is formed and leads are bonded together by laser is disclosed in Japanese Laid-open Patent Publication No. 07-94845. With this semiconductor element mounting board, an end portion of a lead bonded to an electrode pad of the board is thinner than the rest of it.
Moreover, a method for fabricating a semiconductor device including a resin sealed body, first and second semiconductor chips which are inside the resin sealed body and on the front of the front and back of each of which an electrode is formed, a first lead which extends inside and outside the resin sealed body and which is electrically connected to an electrode of the first semiconductor chip, and a second lead which extends inside and outside the resin sealed body and which is electrically connected to an electrode of the second semiconductor chip is disclosed in Japanese Laid-open Patent Publication No. 2006-74073. With this method, the resin sealed body is formed in a state in which the first lead and the second lead overlap. After that, laser welding is performed on each of the first lead and the second lead. This prevents sputters which scatter from reaching a side of each semiconductor chip on which a circuit is formed.
The laser welding illustrated in the above FIG. 9 is performed before injecting resin 17. As illustrated in FIG. 10, when this laser welding is performed, sputters 21 which scatter will short-circuit the circuit patterns 5 and 6 (including a wiring pattern (not illustrated)) and the like formed over the insulating board (ceramic 4), break (blow) a wiring such as the bonding wire 9, or damage the semiconductor chip 8.
That is to say, the scattering of the sputters 21 will cause an insulation failure in the insulating board (ceramic 4), a short circuit between the circuit patterns 5 and 6, breaking of a wire wiring, physical damage (such as a mark of melting, a minute flaw, or a microcrack) to the semiconductor chip 8, and poor electrical properties (such as a short circuit) of the semiconductor chip 8.